1. Field of the Invention
The present invention relates to an automobile antenna control device which is equipped with a microcomputer.
2. Prior Art
Conventional automobile antenna control devices have included devices which use a microcomputer. The operation of such a conventional device is illustrated in FIG. 4 and described as follows:
First, if the starter power source is "on" (S1), extending and/or retracting of the antenna is stopped in midaction (S2). If the starter power source is "off," the status of the radio circuit power source is checked (S3). If the radio circuit power source is "on," the antenna is extended (S4). If the radio circuit power source is "off," the antenna is retracted (S5).
In general, the above mentioned microcomputer is maintained in an actuated state over a long period of time. Accordingly, the microcomputer may be placed in a so-called "runaway" state by an external surge voltages (ordinarily referred to as "external noise") during this period. As a result of this runaway state, the microcomputer becomes uncontrollable. In order to extract the microcomputer from this runaway state, a runaway detecting circuit (referred to as a "watchdog") is installed in conventional devices. When the microcomputer goes into a runaway state, the microcomputer is temporarily reset, so that stable operation is insured.
In the case of general microcomputers which are used in applications other than automobiles, and in which the operation is begun when the power source is switched "on," a power-source-"on" resetting circuit which resets the microcomputer when power begins to be supplied from the power source is sometimes installed, so that normal operation of the microcomputer is insured by this circuit.
However, in the case of the above described conventional devices, the cost of the watchdog is greater than the cost of the microcomputer. As a result, the automobile antenna control device as a whole is increased to a high cost.
In such a case, it would be conceivable to install a resetting circuit constructed with a reset button instead of a watchdog and to have the vehicle operator press this reset button in the case of runaway operation of the microcomputer. In this case, however, the reset button must be installed in a place which ordinarily cannot be reached by the vehicle operator. If this is done, then it is difficult for the vehicle operator to find the reset button when the microcomputer goes into runaway operation. Accordingly, in such cases, the vehicle operator will judge that the antenna control device has malfunctioned, so that maintenance costs are incurred.
Incidentally, since it is sometimes necessary to operate the automobile antenna even when the radio power source is "off," it is not possible to switch the microcomputer power source "on" and "off" simultaneously with the radio power source. Accordingly, the conventional power-source-"on" resetting circuit which resets the microcomputer when the power source is switched "on" cannot be used "as is" in an automobile antenna control device.